Process and apparatus for adsorptive bubble separation

ABSTRACT

Process and apparatus are described for adsorptive bubble separation of hydrophobic particles from liquid dispersions. When a gas-liquid-particle dispersion is introduced into a separation vessel, a baffle directs the rising bubbles toward the perimeter of the apparatus. At the liquid surface, bubbles with attached hydrophobic materials form a floating froth layer, which is directed toward a froth collection launder. Also disclosed is an improvement for froth flotation processes comprising using a vacuum to pull froth and/or collapsed froth into and through the froth collection launder and froth drain line.

REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent application 60/944,813, filed on Jun. 19, 2007, thecontents of the entirety of which are incorporated herein by thisreference.

BACKGROUND

Adsorptive bubble separation (which includes froth flotation, flotation,bubble fractionation, dissolved air flotation, and solvent sublation) isa process in which a molecular, colloidal or particulate material isselectively adsorbed to the surface of gas bubbles rising through aliquid, and is thereby concentrated or separated. A commonly used typeof adsorptive bubble separation process is froth flotation wherein thebubble-particle agglomerates accumulate on the liquid surface as afloating froth. The froth with adsorbed (i.e., attached or collected)particles is treated in one of several ways to collapse the froth andisolate the material. See for example, Flotation Science andEngineering, K. A. Mattis, Editor, pages 1 to 44, Marcel Dekker, NewYork, N.Y., 1995; and Adsorptive Bubble Separation Techniques, RobertLemlich, Editor, pages 1 to 5, Academic Press, New York, N.Y., 1972.

This important process is commercially utilized in a wide range ofapplications including: isolation of minerals and metals from anore-water slurry, dewatering of microalgae, yeast or bacterial cells,removal of oil from water, removal of ash from coal, removal ofparticles in waste-water treatment streams, purification of drinkingwater, and removal of ink and adhesives during paper recycling. In mostapplications, it is necessary to add reagents, known as “collectors”,which selectively render one or more of the species of particles in thefeed hydrophobic, thereby assisting in the process of collection by thegas bubbles. It is also not unusual to add frothing agents to assist inthe formation of a stable froth on the surface of the liquid. Theprocess of admitting these various reagents to the system is known asconditioning. The feed for the adsorptive bubble separation process maybe a mixture, dispersion, emulsion, slurry, or suspension of amolecular, colloidal and/or particulate material in a liquid and isreferred to hereafter as the liquid-particle dispersion. When the liquidis water, as is usually the case, the feed may be referred to as anaqueous-particle dispersion.

Because of the importance of adsorptive bubble separation processes,there have been many attempts to improve the efficiency and selectivityof particle capture from an aqueous-particle dispersion in order toincrease product yield and purity.

U.S. Pat. Nos. 4,668,382, 4,938,865, 5,332,100, and 5,188,726 (thecontents of the entirety of each of which are incorporated herein bythis reference) disclose an adsorptive bubble separation process andapparatus (commonly known as a “Jameson cell”) wherein anaqueous-particle dispersion enters the top of a vertical duct(downcomer) and passes through an orifice plate to form a high velocity,downward facing liquid jet. A gas, usually air introduced into thedowncomer headspace, is dispersed into the mixture as the liquid jetimpacts a foam column within the downcomer. The volume within thedowncomer is referred to as the collection zone wherein most of theparticles adsorb to the surface of the bubbles. The resultinggas-liquid-particle dispersion exits through the bottom of the downcomerinto the separation zone where the bubbles separate from the tails(water and non-adsorbed materials). In the separation zone, thegas-liquid-particle dispersion has sufficient residence time to allowthe tiny bubbles with collected particles to coalesce (combine andenlarge) and rise to the liquid surface forming a particle-rich,floating froth in the froth zone. The froth is collected by allowing itto float outward to the perimeter of the apparatus and overflow into anopen launder (collection trough). Provisions are made in these patentsto incorporate froth washing in the froth zone by introducing a liquidonto the froth from above thus creating a net downward liquid flow andwashing the entrained gangue (undesired solid matter) and non-adsorbedparticles away from the froth. This washing produces a purer froth, andtherefore a more selective separation. In the design described in thesepatents, the washing occurs over the whole surface of the froth ratherthan in a focused region of the froth.

In addition, U.S. Pat. No. 4,668,382 (the contents of the entirety ofwhich are incorporated herein by this reference) changes theconfiguration from a tank with vertical walls to converging walls sothat the froth is squeezed (crowded) as it collects on the liquidsurface. This allows for a higher froth depth than would normally occur,thus permitting better collection selectivity in the portion of frothoverflowing into the collection launder. This design however requires anexpensive fabrication process to make the converging sides.

U.S. Pat. No. 6,832,690 (the contents of the entirety of which areincorporated herein by this reference) also describes a method ofsqueezing the froth in a. complex geometry, while U.S. Pat. No.5,251,764 (the contents of the entirety of which are incorporated hereinby this reference) describes a complex hydraulically-operated system.Froth zone surface fouling can be troublesome in these modifications ofthe original Jameson cell design.

In column flotation cells such as the MICROCEL™, U.S. Pat. Nos.4,981,582 and 5,167,798; the Deister Column Cell, U.S. Pat. No.5,078,921; and the Multistage Loop-Flow Flotation (MSTLFLO) column, U.S.Pat. No. 5,897,772 (the contents of the entirety of each of which areincorporated herein by this reference), the collection, separation, andfroth zones and froth washing are combined in a tall, cylindrical tank,which is less effective and more expensive to construct. In these columnflotation cells, the froth at the top of the column overflows into anouter launder that surrounds the column. Sometimes an additional centrallaunder is added to increase the froth discharge area when it isnecessary to achieve rapid removal of voluminous froth.

Mechanical flotation cells typically employ a rotor and stator mechanismfor gas induction, bubble generation, and liquid circulation thusproviding for bubble and particle collision. The ratio of vessel heightto diameter, termed the “aspect ratio”, usually varies from about 0.7 to2. Typically, four or more cells each having a centrally mounted rotorand stator mechanism are arranged in series. The liquid-particledispersion is fed into the cell and air is sucked into the cell througha hollow shaft agitator. The air stream is broken by the rotatingimpeller, so that small bubbles are emitted from the end of the impellerblades. An auxiliary blower may also be used to provide sufficient gasflow to the cell. Rising bubbles together with attached particles form afroth layer on the top of the liquid surface. The froth layer overflowsor is skimmed off mechanically from the top. Non-floated components arewithdrawn from the bottom of the cell. Mechanical flotation cells areoften used in mineral processing systems; however they have thedisadvantage of large space requirements, long liquid residence times,and high power consumption.

For example, U.S. Pat. Nos. 4,425,232 and 4,800,017 (the contents of theentirety of which are incorporated herein by this reference) describemechanical flotation separation utilizing a flotation cell provided witha rotor-stator assembly submerged in a slurry and in which rotor bladesagitate the slurry thoroughly mixing the solids and liquid andintroducing air to the mixture for aeration and generation of froth onthe liquid surface. Particles of minerals attach to carrier air bubbleswhich are naturally buoyant and form the froth, this being the effectivemechanism for mineral recovery. The floating froth is removed from thetop of the slurry together with the attached mineral particles which arerecovered as froth is collapsed and dewatered.

In all of these previously described processes, the desired particlesthat have prematurely disengaged (i.e., desorbed or detached) from thebubbles are inefficiently contacted with rising gas bubbles over theentire cross sectional area of the tank, thus lowering the chance ofrecapturing them. In addition, these designs typically have frothcollection launders around the perimeter, which reduces the frothdensity as the froth spreads from the center outward (from low surfacearea to high surface area) thereby reducing the froth height and theselectivity of froth overflow.

SUMMARY OF THE INVENTION

Described is a highly efficient process and apparatus (flotation cell)for increasing the collection effectiveness of bubbles and improving thepurity of the froth produced in an adsorptive bubble separation process,in which collected hydrophobic materials (particles) are attached to thebubbles. These materials typically include solids, liquids, or both.Above the froth-liquid interface within the flotation cell is the frothzone wherein the bubble-particle agglomerates form a floating frothlayer. By the design disclosed herein, this froth naturally floatstoward an open central froth collection launder into which it overflows.The action of the rising bubbles at the perimeter pushing the frothlayer toward the reduced surface area of the center squeezes (crowds)the froth causing bubble coalescence and increased liquid drainagethereby achieving an increased concentration of collected materials.

The improved adsorptive bubble separation design may be utilized in anyflotation cell by forcing floating froth to flow on the liquid surfaceto a region of lower surface area before overflowing into a collectionlaunder. This improved froth collection design may be utilized in theoperation of mechanical flotation cells, pneumatic flotation cells suchas the Jameson cell, Multistage Loop-Flow flotation columns, and bubbleflotation columns (also known as “Canadian Columns”) by replacement ofthe their perimeter collection launder with a central collectionlaunder.

As a consequence of the design, the length of the collection launder lip(referring to the launder edge which the froth overflows) is shorterthan the length of the perimeter of the separation apparatus. This is incontrast to processes of the prior art wherein the collection launder islocated around the perimeter of the apparatus so that the launder liplength is the same length as the perimeter. In those prior art designswhere a central launder is also used, the launder lip length is furtherincreased. The design of the invention is especially useful in therecovery or removal of low concentrations of hydrophobic materials inwater. In oil recovery from water, for example, it is desirable toconcentrate the oil in the froth to the greatest extent possible beforeit is removed from the flotation cell. This design is also useful forthe dewatering of microalgae in very dilute microalgal cultures.

In any adsorptive bubble separation process, a portion of the desiredhydrophobic material is not captured by, or is dislodged from, therising bubbles. An optional performance enhancement in the instantdesign provides a means for forcing re-contact of these disengagedparticles with rising bubbles at the perimeter. This enhancement isachieved by the use of a baffle in the separation vessel which causesthe disengaged particles to flow down and outward with the liquiddraining from the froth to re-contact the rising bubbles at theperimeter. This re-contact with bubbles encourages re-adsorption of thedesired hydrophobic material resulting in better recovery than generallyobtained in the prior art.

The process for adsorptive bubble separation can be repeated one or moretimes in order to affect an efficient countercurrent flow of froth anddraining liquid for highly efficient particle capture. The process foradsorptive bubble separation can be operated batchwise or continuously.Continuous operation is preferred in most applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic block diagram of the process.

FIG. 2 is a sectional view of a cylindrical separation apparatus inwhich the gas-liquid-particle dispersion introduction ducts are arrangednear the perimeter of the vessel and the froth collection launder is inthe center.

FIG. 3 is a top view of the apparatus of FIG. 2.

FIG. 4 is a sectional view of a cylindrical separation apparatus inwhich the gas-liquid-particle dispersion introduction ducts are arrangednear the perimeter of the vessel, the froth collection launder is in thecenter and a baffle is used to direct disengaged particles back to theperimeter for re-contact with rising bubbles.

FIG. 5 is a sectional view of a cylindrical separation apparatus inwhich a single gas-liquid-particle dispersion introduction duct islocated in the center and the central froth collection launder isattached to the duct in an annular fashion, a baffle is used to directthe rising bubbles to the perimeter.

FIG. 6 is an illustration of how a conventional bubble column can bemodified with a central froth launder to utilize the design of theinvention.

FIG. 7 is an illustration of how a mechanical flotation cell can bemodified with a central froth launder to utilize the design of theinvention.

FIG. 8 is a sectional view illustrating a rectangular separationapparatus with gas-liquid-particle dispersion ducts arranged on thediagonal and froth collection launders in the opposite corners.

FIG. 9 is a top view of the rectangular apparatus of FIG. 8.

FIG. 10 is a sectional view illustrating an optional vacuum-aided frothcollection system and an optional spray-aided froth collection system.

FIG. 11 is a top view of a cylindrical flotation cell of the type shownin FIGS. 2 and 3 which illustrates how the floating froth crowds intoareas of lower surface area before spilling into the central frothcollection launder.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIG. 1, the froth flotation device includes a bubblegeneration zone 3, a collection zone 4, a separation zone 5, and a frothzone 7. Some or all of these zones may or may not occupy the samevessel. The liquid-particle dispersion feed 1 enters the froth flotationdevice at either the collection zone 4 or the bubble generation zone 3or both, depending on the equipment chosen. In either event, a gas 2 isdispersed through the bubble generation zone 3 and/or the collectionzone 4 to produce a gas-liquid-particle dispersion. It is desirable toproduce a large number of small bubbles to maximize the surface area ofgas available for collision with hydrophobic particles in a given volumeof the feed dispersion 1.

In the collection zone 4, the hydrophobic particles are mixed with thefine bubbles under conditions that promote intimate contact to producethe gas-liquid-particle dispersion. The bubbles collide with thehydrophobic particles and form bubble-particle agglomerates. It isdesirable to generate intense mixing in the collection zone 4 to cause ahigh frequency of collisions in order to achieve high particle captureefficiency.

After the bubble-particle agglomerates are formed in the collection zone4, they are then separated from the particle-depleted liquid in theseparation zone 5, typically by gravity. The density of the gas isgenerally at least two to three orders of magnitude less than that ofthe liquid. The density difference promotes floating of thebubble-particle agglomerates to the surface of the liquid, where theagglomerates accumulate as a froth in the froth zone 7.

The froth, enriched in hydrophobic particles, overflows the froth zone 7as stream 8. The underflow stream (tails) 6, which is the liquiddepleted in hydrophobic particles, exits the froth flotation device andmay be treated again in a secondary flotation cell, recycled ordiscarded.

FIG. 2 shows an apparatus for adsorptive bubble separation in which thegas-liquid-particle dispersion 10 is introduced below the liquid surface(froth-liquid interface) 11 through introduction ducts 12 into theseparation zone 13. In all embodiments of this invention the gas maycomprise air, nitrogen, argon, helium, carbon dioxide, gas from thecombustion of carbonaceous material, solvent vapor, carbon dioxide froma gasification plant, or combinations thereof. The gas may also bepre-saturated with liquids, especially liquids that are contained in thefeed dispersion. The gas-liquid-particle dispersion 10 can be producedby methods known in the prior art such as aspiration of the gas into theliquid-particle dispersion using an eductor, a plunging jet, or anagitated system.

The introduction ducts 12 introducing the gas-liquid-particle dispersion10 may be vertical, have a vertical section, or be essentially verticalso that it contains the column of foam in a manner so that sufficientvacuum is maintained in the top of the duct to maintain a column of atleast some of the gas-liquid-particle dispersion in the duct.Alternatively (or additionally), the introduction ducts may enter theflotation cell through the side of the vessel. This embodiment may alsohave two or more introduction ducts 12 evenly spaced near the perimeterof the vessel. It is preferred to have four or more introduction ducts12 evenly spaced near the perimeter of the vessel.

The gas-liquid-particle dispersion 10 exiting from the introductionducts 12 has been created such that the liquid-particle dispersion feedhas been brought into intimate contact with the gas with enough energyand for sufficient time that an acceptable percentage of the hydrophobicmaterial has been collected on the bubbles. The introduction ducts 12may be cylindrical in shape, but other geometries could be used,including but not limited to rectangles, squares, ovals, triangles, andother polygons. The introduction ducts 12 may be constructed from anymaterial used in the art, including but not limited to polyvinylchloride (PVC), high-density polyethylene (HDPE), polycarbonate, otherpolymers, glass, fiberglass, steel, iron, other metals, concrete, tile,or other construction materials.

The bottoms of the introduction ducts 12 are submerged below the surfaceof the liquid 11 within the separation zone 13 where the exitinggas-liquid-particle dispersion 10 begins to coalesce into larger bubbles14 and rise toward the liquid surface 11 carrying the collectedmaterials. The separation zone 13 can be configured in any shape so longas the residence time is sufficiently great to allow for bubblecoalescence and the separation of the bubble-particle agglomerates andliquid stream. Even though the separation zone 13 may be cylindrical,square, rectangular, hexagonal, or other shape, it is preferable to usea cylindrical design. The outer wall 15 of the flotation cell may beconstructed from any material used in the art, including but not limitedto polyvinyl chloride (PVC), high-density polyethylene (HDPE),polycarbonate, other polymers, glass, fiberglass, steel, iron, othermetals, concrete, tile, earth, stone, or other construction materials.

The rising bubble-particle agglomerates 14 accumulate as a froth 17above the liquid surface (froth-liquid interface) 11 in the froth zone16. In this zone, the froth continues to drain, purifying the froth andconcentrating the collected material. As additional bubbles rise formingmore froth, they push the accumulated froth on the surface toward thecenter where the upper portion overflows the launder lip into the frothcollection launder 9. This movement toward the reduced surface area ofthe center squeezes the froth helping to cleanse and drain it. Washwater may be added to the froth from above if desired to further purifyit. Any suitable liquid can be used for the froth washing operation.Suitable liquids include, but are not limited to water, liquids that arenative to the feed dispersion, solutions of surface treatment andconditioning agents, and combinations thereof.

The purified froth 17 overflows into a central collection launder 9 thatextends above the liquid surface level 11. The collection launder 9 canbe of any shape, but it is preferably the same geometric shape as theflotation cell. The central collection launder 9 is most preferably acircular pipe or hollow column. The collection launder 9 may beconstructed from any material used in the art, including but not limitedto polyvinyl chloride (PVC), high-density polyethylene (HDPE),polycarbonate, other polymers, glass, fiberglass, steel, iron, othermetals, concrete, tile, or other construction materials. The froth andcollapsed froth 18 drains down the collection launder 9 and then exitsthe flotation cell through the bottom or the side via a drain line 19.

The feed liquid depleted in hydrophobic particles (tails) 20 underflowsthe flotation device through a bottom or side tails line 21 and may betreated again in a secondary flotation cell, recycled or discarded. Thebottom of the flotation cell may be flat, hemispherical or conical. Inprocesses in which solids settle to the bottom, a sloped-flat,hemispherical or conical bottom with bottom tails line 21 is desired forimproved solids removal. The liquid level 11 within the flotation cellmay be controlled by controlling the liquid flow through the tails line21. Optionally, liquid level control may be conveniently maintainedwithout valves or control devices by the use of one or more overflowside arms or swing arms.

FIG. 3 shows a top view of the apparatus for adsorptive bubbleseparation of FIG. 2 employing four introduction ducts 12.

FIG. 4 shows an optional performance enhancement to the apparatus foradsorptive bubble separation of FIGS. 2 and 3. This optional enhancementincludes a baffle 23 to direct the disengaged particles to flow down andoutward with the liquid draining from the froth to re-contact the risingbubbles at the perimeter. This re-contact with bubbles encouragesre-adsorption of the desired hydrophobic material resulting in betterrecovery than would be obtained by methods in the prior art.

A froth baffle 23 directs the bubbles 24 outward to the gap 25 betweenthe baffle 23 and the perimeter wall 26 where they then rise toward theliquid surface 27. Likewise disengaged particles that are sinking aredirected outward by baffle 23 to the gap 25 where they will bere-contacted with rising bubbles. The froth baffle 23 may be of anyshape which directs the rising bubbles and the sinking particles to alocation near the perimeter. The baffle 23 may be conical, flat,tapered, or sloped and constructed from any material used in the art,including but not limited to PVC, HDPE, polycarbonate, rubber, otherpolymers, glass, fiberglass, steel, iron, other metals, concrete, tile,or other construction materials.

FIG. 5 shows an apparatus for adsorptive bubble separation in which thegas-liquid-particle dispersion 28 is introduced below the liquid surface29 through one central introduction duct 30 into the separation zone 31.A froth baffle 32 directs the bubbles 33 outward to the gap 34 betweenthe baffle 32 and the perimeter wall 35 where they then rise toward theliquid surface 29. Likewise disengaged particles 36 that are sinking aredirected outward by baffle 32 to the gap 34 where they will bere-contacted with rising bubbles. The froth baffle 32 may be of anyshape which directs the rising bubbles and the sinking particles to alocation near the perimeter. The baffle may be conical, flat, tapered,or sloped and constructed from any material used in the art, includingbut not limited to PVC, HDPE, polycarbonate, rubber, other polymers,glass, fiberglass, steel, iron, other metals, concrete, tile, or otherconstruction materials.

The rising bubble-particle agglomerates accumulate, as a froth 37, abovethe liquid surface 29 in the froth zone 38. In this zone, the frothcontinues to drain, purifying the froth and concentrating the collectedmaterial. As additional bubbles rise forming more froth, they push theaccumulated froth on the surface toward the center where the upperportion overflows the launder lip into the collection launder 39. Thismovement toward the reduced surface area of the center squeezes thefroth helping to cleanse and drain it. Wash water may be added to thefroth from above if desired to further purify it.

The purified froth 37 overflows into a central collection launder 39which extends above the liquid surface level 29. The collection launder39 can be of any shape, but it is preferred that the collection launder39 be an annulus around a tubular introduction duct 30. The collectionlaunder 39 may be constructed from any material used in the art,including but not limited to polyvinyl chloride (PVC), high-densitypolyethylene (HDPE), polycarbonate, other polymers, glass, fiberglass,steel, iron, other metals, concrete, tile, or other constructionmaterials. The froth or collapsed froth 37 drains down the collectionlaunder 39 and then exits the flotation cell through the bottom or theside via a drain line 40.

The feed liquid depleted in hydrophobic particles (tails) 41 underflowsthe flotation device through a bottom or side tails line 42 and may betreated again in a secondary flotation cell, recycled or discarded. Thebottom of the flotation cell may be flat, hemispherical or conical. Inprocesses in which solids settle to the bottom, a sloped-flat,hemispherical or conical bottom with bottom tails line 42 is desired forimproved solids removal. The liquid level 29 within the flotation cellmay be controlled by controlling the liquid flow through the tails line42. Optionally, the liquid level 29 may be maintained by the use of oneor more self-leveling devices such as an overflow side arm or swing arm.

One of the design parameters in flotation cell design is Jg (thesuperficial gas rise rate), and it is typically calculated by dividingthe gas flow rate entering the cell by the cell area. High Jg rates(greater than 1 cm per second) will typically produce high recovery as arapid bubble rise rate leaves less time for the bubbles to coalesce andparticles to disengage. In this rising stream, gangue and liquid can beentrained in the flow also. Lower Jg rates (less than 1 centimeter persecond) allow more time for bubble coalescence and drainage of the frothto produce a more pure froth. An even rise of bubbles is assumed in thecalculation of Jg. In the prior art, it is understood that uniformity inthe froth flow path results in uniform treatment of the froth and givesmore predictable performance. With a central froth baffle in acylindrical tank, froth distribution takes place in a radial directionwith constant distances and a uniform inward froth flow path throughout360 degrees.

FIG. 6 shows a flotation column embodiment of the invention in which thecollection, separation, and froth zones and optional froth washing arecombined in a tall, cylindrical tank 43. This design is an improvementof the conventional bubble column which is also known as the “CanadianColumn”. The cross section may be circular, square or hexagonal. Theliquid-particle dispersion enters the column at a point below the liquidsurface (froth-liquid interface) 44 through line 45. The gas enters thebase of the column through line 46 and is dispersed into fine bubbles,typically by means of a sparger 47 or it is introduced as an aeratedliquid.

The countercurrent flow of gas and feed dispersion results in bubble andparticle collision in the collection zone 48 which is defined as theregion below the feed distributor 49. The separation zone 50 for thecolumn is above the feed distributor 49 and below the froth-liquidinterface 44.

The rising bubble-particle agglomerates accumulate as a froth 51 abovethe liquid surface 44 in the froth zone 52. In this zone the frothcontinues to drain, purifying the froth and concentrating the collectedmaterial. As additional bubbles rise forming more froth, they push theaccumulated froth on the surface toward the reduced surface area of thecenter where the upper portion overflows the launder lip into thecollection launder 53. This movement toward the center squeezes thefroth helping to cleanse and drain it. Wash water may be added to thefroth from above if desired to further purify it.

The purified froth 51 overflows into a central collection launder 53which extends above the froth-liquid interface 44. The collectionlaunder 53 can be of any shape, but it is preferably the same geometricshape as the flotation cell. The central collection launder 53 is mostpreferably a circular pipe or hollow column. The collection launder 53may be constructed from any material used in the art, including but notlimited to polyvinyl chloride (PVC), high-density polyethylene (HDPE),polycarbonate, other polymers, glass, fiberglass, steel, iron, othermetals, concrete, tile, or other construction materials.

The froth or collapsed froth 51 drains down the collection launder 53and then exits the flotation cell through the bottom or the side via adrain line 54. The tails, depleted of hydrophobic particles, underflowsthe column through a bottom or side tails line 55 and may be treatedagain in a secondary flotation cell, recycled or discarded. The bottomof the flotation cell may be flat, hemispherical or conical. Inprocesses in which solids settle to the bottom, a sloped-flat,hemispherical or conical bottom with bottom tails line 55 is desired forimproved solids removal. The liquid level 44 within the flotation cellmay be controlled by controlling the liquid flow through the tails line55. Optionally, the liquid level 44 may be maintained by the use of oneor more self-leveling devices such as an overflow side arm or swing arm.

FIG. 7, shows a mechanical flotation cell embodiment of the invention inwhich the bubble generation zone 57, collection zone 58, separation zone59, and froth zone 60 are combined into a single large tank 56.Mechanical cells typically employ a rotor and stator mechanism 61 forgas induction, bubble generation, and liquid circulation providing forbubble and particle collision. Typically, four or more cells similar tothat in FIG. 7, each having a centrally mounted rotor and statormechanism 61, are arranged in series to improve efficiency. An auxiliaryblower is occasionally installed to provide sufficient gas flow to thecell.

The gas is dispersed into fine bubbles by a rotating impeller 62, whichserves as the bubble generator. The rotating impeller creates a lowpressure zone that induces gas to flow through an aspiration tube 63into the collection zone 58 where it is dispersed into fine bubbles andmixed with the liquid-particle dispersion as it circulates in the bottomof the cell.

The properly designed rotor and stator mechanism entrains the properamount of gas, disperses it into fine bubbles, and mixes the gas withliquid to accomplish sufficient contact between the particles and thebubbles. Good mixing and sufficient liquid residence time are necessaryin the two phase mixing region to provide high bubble and particlecollision efficiency, and good flotation performance. Rotor and statormechanisms include those produced by Dorr-Oliver Incorporated ofMillford, Conn.; Denver Equipment Company which is a division of Svedalaof Colorado Springs, Colo.; Wemco Products of Salt Lake City, Utah; andOutomec Oy of Espoo, Finland.

The liquid dispersion enters the mechanical cell as a feed stream 64through a feed box 65. Bubble and particle contact results fromturbulence generated by the rotating impeller 62. The bubbles withattached particles pass out of the collection zone 58 into theseparation zone 59, which is relatively quiescent, where they float tothe surface and separate from the liquid phase.

The rising bubbles accumulate as a froth 66 above the liquid surface(froth-liquid interface) 67 in the froth zone 60. In this zone the frothcontinues to drain, purifying the froth and concentrating the collectedmaterial. As additional bubbles rise forming more froth, they push theaccumulated froth on the surface toward the reduced surface area of thecenter where the upper portion overflows the launder lip into thecollection launder 68. This movement toward the center squeezes thefroth helping to cleanse and drain it. Wash water may be added to thefroth from above if desired to further purify it.

The purified froth 66 overflows into a central collection launder 68that extends above the liquid surface level 67. The collection launder68 can be of any shape, but it is preferred that the collection launder68 be an annulus around a circular aspiration tube 63 and rotor shaft69. The collection launder 68 may be constructed from any material usedin the art, including but not limited to PVC, HDPE, polycarbonate, otherpolymers, glass, fiberglass, steel, iron, other metals, concrete, tile,or other construction materials. The froth or collapsed froth drainsdown the collection launder 68 and then exits the flotation cell throughthe bottom or the side via a drain line 70.

The liquid phase recirculates in the collection zone 58, but eventuallyexits the cell as an underflow stream 71 through a bottom or side tailsline 72. The tails may be treated again in a secondary flotation cell,recycled or discarded. The bottom of the flotation cell may be flat,hemispherical or conical. In processes in which solids settle to thebottom, a sloped-flat, hemispherical or conical bottom with bottom tailsline 72 is desired for improved solids removal. The liquid level 67within the flotation cell may be controlled by controlling the liquidflow through the tails line 72. Optionally, the liquid level 67 may bemaintained by the use of one or more self-leveling devices such as anoverflow side arm or swing arm.

FIG. 8, shows an example of how our discovery for improving froth purityby causing the froth to squeeze into a region of lower surface area canbe utilized in alternate flotation cell geometries. FIG. 8 shows arectangular, or preferably square, flotation cell embodiment of theinvention in which the gas-liquid-particle dispersion 73 is introducedbelow the liquid surface (froth-liquid interface) 74 through two or moreintroduction ducts 75 into the separation zone 76. It is preferred inthis embodiment to have two or more introduction ducts 75 evenly spacedalong a diagonal of the vessel. The introduction ducts 75 may beconstructed from any material used in the art, including but not limitedto polyvinyl chloride (PVC), high-density polyethylene (HDPE),polycarbonate, other polymers, glass, fiberglass, steel, iron, othermetals, concrete, tile, or other construction materials.

The bottoms of the introduction ducts 75 are submerged below the surfaceof the liquid 74 within the separation zone 76 where the exitinggas-liquid-particle dispersion 73 begins to coalesce into larger bubblesand rise carrying the collected materials. The separation zone 76 inthis embodiment is square or rectangular and of such height as to allowfor bubble coalescence and the separation of froth and liquid streams.The outer wall 77 of the flotation cell may be constructed from anymaterial used in the art, including but not limited to PVC, HDPE,polycarbonate, other polymers, glass, fiberglass, steel, iron, othermetals, concrete, tile, earth, stone, or other construction materials.

The rising bubble-particle agglomerates accumulate as a froth 78 abovethe liquid surface (froth-liquid interface) 74 in the froth zone 79. Inthis zone the froth continues to drain, purifying the froth andconcentrating the collected material. As additional bubbles rise formingmore froth, they push the accumulated froth on the surface toward thetwo corners where the upper portion overflows the launder lips into thecollection launders 80. This movement toward the lower-surface-areacorners squeezes the froth helping to cleanse and drain it. Wash watermay be added to the froth from above if desired to further purify it.

The purified froth 78 overflows into the corner collection launders 80which extend above the liquid surface level 74. The collection launders80 may be any shape although shapes that fit tightly in the corner arepreferred. These shapes would include square, rectangular, triangular,quarter-circle and circular. The collection launders 80 may beconstructed from any material used in the art, including but not limitedto PVC, HDPE, polycarbonate, other polymers, glass, fiberglass, steel,iron, other metals, concrete, tile, or other construction materials. Thefroth or collapsed froth 78 drains down the collection launders 80 andthen exits the flotation cell through the bottom or the side via drainlines 81. Alternatively, the collection launders in this rectangularembodiment may also be exterior to the flotation cell by the use of acorner notch which serves as the froth collection launder lip.

The feed liquid depleted in hydrophobic particles (tails) 82 underflowsthe flotation device through a bottom or side tails line 83 and may betreated again in a secondary flotation cell, recycled or discarded. Thebottom of the flotation cell may be flat, hemispherical or conical. Inprocesses in which solids settle to the bottom, a sloped-flat,hemispherical or conical bottom with bottom tails line 83 is desired forimproved solids removal. The liquid level 74 within the flotation cellmay be controlled by controlling the liquid flow through the tails line83. Optionally, liquid level control may be conveniently maintainedwithout valves or control devices by the use of one or more overflowside arms or swing arms.

FIG. 9 shows a top view of the flotation cell embodiment of FIG. 8employing three introduction ducts 75.

In the process of the invention the feed dispersion comprises particlesand a carrier fluid, usually water. The particles may comprise solidparticles or liquid droplets, or combinations thereof. Examples of thesolid particles include, but are not limited to minerals, gangue,micro-organisms, coal, inks, pigments, or combinations thereof. Examplesof liquid droplets include, but are not limited to organic solvents,metal extraction solvents, dyes, inks, oils, hydrocarbons, fuels,triglycerides, carotenoids, natural products, biodiesel, or other fluidsthat are above their melting point and below their boiling point at thesystem pressure and temperature. Practical examples of the feeddispersions requiring separation include but are not limited to mineralsand gangue, aqueous dispersions of micro-organisms (microalgae,bacteria, fungi, and/or viruses), aqueous dispersions of oil dropletsand unwanted particulates, aqueous dispersions of triglycerides, coaland unwanted materials (e.g., ash), particles in waste-water treatmentstreams, and inks and/or adhesives on paper for recycling, orcombinations thereof. Micro-organisms in the feed may be alive or dead,whole or ruptured. The apparatus and process of this invention isespecially useful for the concentration (dewatering) of rupturedmicroalgal cells and microalgal cellular components in water. Additivesmay be introduced to facilitate flotation of micro-organism cells suchas alum, ferric chloride, poly-electrolytes, polymers, and otherflocculants known in the art. The carrying liquid may be water, brine,seawater, aqueous solutions, growing media for the microalgae orreagents or a combination of any of these.

In biochemical process engineering, adsorptive bubble separation findsutility in isolation or concentration of valuable natural products suchas are produced by, for example, microalgae. Often in such applicationsthe desired organism or biochemical product is present in very lowconcentration. In such cases it is necessary therefore to feed largevolumes of a very dilute aqueous dispersion of the desired material. Seefor example, “Harvesting of Algae by Froth Flotation,” G. V. Levin, etal., Applied and Environmental Microbiology, volume 10, pages 169-175(1962). U.S. Pat. Nos. 5,776,349 and 5,951,875, the contents of whichare incorporated herein by this reference, disclose the use of a Jamesoncell for dewatering an aqueous dispersion of ruptured microalgae cells.

The microalgae can be any species of microalgae one desires to separatefrom the carrying liquid. These species include, but are not limited toAnabaena, Ankistrodesmus falcatus, Arthrospira (Spirulina) obliquus,Arthrospira (Spirulina) platensis, Botryococcus braunii, Chaetocerosgracilis, Chlamydomonas reinhardtii, Chlorella vulgaris, Chlorellapyrenoidosa, Chlorococcum littorale, Cyclotella cryptica, Dunaliellabardawil, Dunaliella salina, Dunaliella tertiolecta, Dunaliella viridis,Euglena gracilis, Haematococcus pluvialis, Isochrysis galbana,Nannochloris, Nannochloropsis sauna, Navicula saprophila, Neochlorisoleoabundans, Nitzschia laevis, Nitzschia alba, Nitzschia communis,Nitzschia paleacea, Nitzschia closterium, Nostoc commune, Nostocflagellaforme, Pleurochrysis carterae, Porphyridium cruentum,Prymnesium, Pseudochoricystis ellipsoidea, Scenedesmus obliquus,Scenedesmus quadricauda, Scenedesmus acutus, Scenedesmus dimorphus,Skeletonema costatum, Spirogyra, Spirulina, Synechoccus, Amphora,Fragilaria, Schizochytrium, Rhodomonas, and genetically-engineeredvarieties of these microalgal species. It should be understood that anadditional reason for the separation of microalgae may be to clean thecarrying liquid, rather than just for the purpose of concentratingmicroalgal biomass.

In mineral processing applications, the feed dispersion is conditionedusing surface chemistry treatments as are known in the art that renderthe desired mineral hydrophobic. When the feed dispersion is contactedwith gas, the hydrophobic materials attach and rise with the bubbles.The undesired gangue material then flows downward with the liquid. Thisinvention is also useful in those instances where the undesired materialis rendered hydrophobic and is removed with the froth. In such cases theunderflow contains the desired material.

Due to the difficulty in transporting froths in ducts and pipes, it iscritical to collapse the purified froth in the collection launder. Thepurified froth which overflows into the collection launder naturallycollapses or is treated in one of several ways known in the art tocollapse the froth and isolate the concentrated material. With morepersistent froths, more aggressive action must be taken to collapsethem. The use of froth sprays in the froth collection launder is commonin the prior art. The liquid used for the sprays can be water or anyother liquid. So as to not dilute the collected material, the liquidportion of the collapsed froth can be recirculated through the spraynozzles. It is important that the spray nozzles be of a design toprevent clogging. Air or gas selected from those used for creation ofthe foam can also be used to break the froth.

Persistent froths (which do not collapse easily) are typically brokendown by the use of sprays of liquid into the collection launders afterthe froth has overflowed into them. When the froth is allowed to flow tothe perimeter of the separation vessel for collection in launders, thearea needing treatment is the entire perimeter of the vessel. Thisaddition of spray liquid can be large thus diluting the froth andpartially defeating the purpose of the operation. This dilution isundesirable because adsorptive bubble separation is commonly used forthe concentration of a hydrophobic material. Therefore, the instantinvention provides an improvement in that the central collection launderhas a lower surface area and requires less spray volume.

Chemical methods for breaking froth are also known in the prior art andinclude, but are not limited to the use of chemical defoamers. Thesedefoamer liquids can be sprayed on the froth surface or distributedwithin a wetted pad that contacts the froth as it flows into the frothcollection launder. These chemical methods are also applicable for usein the instant invention and the lower surface area of the centralcollection launder may result in the use of less chemical defoamer.

Mechanical methods to break the froth include, but are not limited tosonication methods, vibrating or spinning objects in the froth region,etc. Also, combinations of chemical and mechanical coalescing techniquescan be used to coalesce the bubbles and form a region enriched in theparticles to be separated from the liquid stream. These mechanicalmethods are also applicable to use in the instant invention and thelower surface area of the central collection launder may use smaller andless expensive equipment.

Described are improved froth flotation separation processes that usepartial vacuum (i.e., suction or downdraft) to pull the froth and/orcollapsed froth into and through the froth collection launder and frothdrain line. This improvement greatly assists froth collection(especially with persistent froths) because froth can be pulled bysuction through the collection system more easily than it can be drainedby gravity or pushed. This partial vacuum or suction can be generated byany method known in the art, but it is particularly useful if it iscreated as a result of the need to supply gas to generate thegas-liquid-particle dispersion. By supplying the gas for thegas-liquid-particle production from the head space of a tank receivingthe collected froth and collapsed froth, the partial vacuum or suctionis created. This vacuum-aided froth collection embodiment is illustratedin FIG. 10 in integration with the apparatus previously described inFIG. 2 but it may be used in any flotation cell using a froth collectionlaunder.

The vacuum source for this optional performance enhancement may begenerated by the gas blower or compressor 84 which provides theflotation gas 85 either for sparging into the flotation cell or formixing with the liquid-particle dispersion 86 to give thegas-liquid-particle dispersion 87. This may be achieved by the use of acollapsed froth trap 88 from which the gas blower or compressor 84obtains its supply of gas 89 via supply line 90. Alternatively thevacuum can be self-generated by the use of an aspirating aerator (e.g.,aspirator, venturi or eductor) 91 which is used to create thegas-liquid-particle dispersion 87. In the case of self aspiratingaerators, no gas blower or compressor 84 is needed but the vacuum isprovided by the Venturi effect of the aspirating aerator 91. Otherbenefits of this vacuum-aided froth collection embodiment are thepre-saturation of the flotation gas with the liquid in use and theability to use a closed flotation cell with an inert gas.

A second optional enhancement, spray-aided froth collection, illustratedin FIG. 10, may be used with or without the previous vacuum-aided frothcollection. In this optional enhancement, a spray is used to improvefroth collection. Pump 92 is used to generate the spray 93 fromcollapsed froth liquids 94 in trap 88. The spray 93 drives thepersistent froth into the froth collection launder 95. The use ofcollapsed froth liquids for this spray prevents dilution of thecollected materials.

In the case of particulate flotation, where the gangue material isdenser than the liquid in the feed dispersion, a sloped bottom and asolids relief discharge will be needed to remove solids from theseparation vessel. If the feed rate of the feed dispersion is somewhatconstant, this solids relief discharge can be controlled by the use of asmall valve set to discharge from the sloped bottom and remove thesolids in a heavy slurry. In another aspect, this removal could bethrough a small solids handling pump.

Any suitable liquid can be used for the optional froth washingoperation. Suitable liquids include, but are not limited to water,liquids that are native to the feed dispersion, solutions of surfacetreatment and conditioning agents, and combinations thereof.

FIG. 11 illustrates the froth crowding effect that occurs as thefloating froth travels from a liquid surface region where it forms to aliquid surface region of lower area before spilling into the collectionlaunder. This top view of a cylindrical flotation cell 96 of the typeshown in FIGS. 2 and 3 emphasizes how the froth in, for example, region97 must crowd into region 98 then region 99 before spilling into thecentral froth collection launder 100. Imaginary radial lines 101 and 102and equi-distant concentric circles 103 and 104 are shown forillustration purposes only.

The process and apparatus for adsorptive bubble separation is furtherillustrated by the following Examples.

EXAMPLES Example 1 Extractant Recovery From Copper Extraction Raffinate

A pilot-scale flotation cell is constructed with the design shown inFIG. 5 with the overall dimensions of the cylindrical separation zonesection being 0.3 meters diameter and 1.3 meters in height. The annularclearance between the froth baffle and the outside is 1.25 centimetersand the froth collection launder lip diameter is 11.5 centimeters. Theseparation zone is fed through a 5 centimeter diameter introduction ductwith an aqueous feed dispersion containing 0.02 wt % kerosene (solvent)with 0.005 wt % of Cyanex extractant. The feed dispersion rate is 35liters per minute and the gas (air) flow rate is 8 liters per minute.The Jg in the separation zone is 0.62 centimeters/second, and this showsa moderate transport rate of collected material from the separation zoneinto the froth zone. The bubble-particle agglomerates residence time inthe separation zone is 168 seconds, while the froth residence time inthe froth zone is 562 seconds. Ninety percent of the kerosene fed to theflotation cell is recovered in the froth.

Example 2 Extractant Recovery From Copper Extraction Raffinate

A pilot-scale flotation cell is constructed with the design shown inFIG. 5 with the overall dimensions of the cylindrical separation zonebeing 0.3 meters diameter and 1.3 meters in height. The annularclearance between the froth baffle and the outside is 1.25 centimetersand the froth collection launder lip diameter is 8.9 centimeters. Theseparation zone is fed through a 7.5 centimeter diameter duct with anaqueous feed dispersion containing 0.02 wt % kerosene with 0.005 wt %Cyanex extractant. The feed rate is 53 liters per minute and the gas(air) flow rate is 13 liters per minute. The Jg in the separation zoneis 1 centimeter/second, and this shows a high transport rate ofcollected material from the separation zone into the froth zone. Thebubble-particle agglomerates residence time in the separation zone is 96seconds, while the froth residence time in the froth zone is 321seconds. Eighty-five percent of the kerosene fed to the flotation cellis recovered in the froth.

Example 3 Dewatering of an Algal Dispersion

A commercial-size flotation cell is constructed with the design shown inFIG. 5 with the overall dimensions of the cylindrical separation zonebeing 1.25 meters diameter and 2.5 meters in height. The annularclearance between the lower froth baffle and the outside wall is 0.125meters and the froth lip is 0.58 meters from the upper froth baffle. Theseparation zone is fed through a 0.2 meter diameter duct with agas-aqueous dispersion containing 0.05 wt % ruptured Dunaliella salinamicroalgae. The feed rate is 750 liters per minute, and the gas flowrate is 190 liters per minute. The Jg in the separation zone is 0.71centimeters/second, which shows a high transport rate of collectedmaterial from the separation zone into the froth zone. Thebubble-particle agglomerates residence time in the separation zone is117 seconds, while the froth residence time in the froth zone is 390seconds. The collected froth after collapsing shows a twenty-foldincrease in ruptured microalgae concentration.

Example 4 Mineral Recovery

The same pilot-scale flotation cell described in Example 3 is usedherein. A 25 wt % aqueous slurry of copper sulfide ore chalcocite (Cu₂S)particles with a d80 particle size of 80 microns (i.e., 80% of theparticles are less than 80 microns) is fed to the cylindrical separationzone from the central duct. The density of the copper sulfide oreparticles is 5.5 g/ml, and the solids contain 0.1% chalcocite. Theunfloated solids are discharged through a solids relief line at a solidsflow velocity greater than 4 feet/second in order to prevent depositionof the solids and sanding of the line. Thus a high percentage of thetotal liquid underflow flows through the solids relief valve. The meanresidence time of the chalcocite particles in the separation zone is 156seconds, and the mean residence time of the chalcocite particles in thefroth zone is 520 seconds. The chalcocite particles exit in the frothcollection launder with 2 wt % chalcocite and a single-pass recoveryrate of 65%.

Example 5 Dewatering of an Unruptured Microalgal Dispersion

A commercial-size flotation cell as per Example 3 is constructed. Theseparation zone is fed through a 0.2 meter diameter duct with angas-aqueous dispersion containing 0.05 wt % of whole Dunaliella salinamicroalgae together with alum and polymer as is known in the art. Theliquid feed rate is 750 liters per minute, and the gas flow rate is 190liters per minute. The Jg in the separation zone is 0.71centimeters/second, showing a high transport rate of collected materialfrom the separation zone into the froth zone. The bubble-particleagglomerate residence time in the separation zone is 117 seconds, whilethe froth residence time in the froth zone is 390 seconds. The collectedfroth (after collapsing) shows a twenty-fold increase in microalgaeconcentration.

1-14. (canceled)
 15. An adsorptive bubble separation process of the typeinvolving froth flotation wherein the floating froth flows into acollection launder, wherein the improvement comprises: directing risingbubbles and falling dislodged particles to the perimeter of a flotationchamber with one or more baffles; and forcing the floating froth to flowon the liquid surface to a region of reduced surface area beforeoverflowing into the collection launder.
 16. An adsorptive bubbleseparation process of the type involving froth flotation wherein thefloating froth flows into a collection launder, wherein the improvementcomprises: directing rising bubbles and falling dislodged particles tothe perimeter of a flotation chamber with one or more baffles; andutilizing a vacuum to pull and/or collapse froth through the collectionlaunder.
 17. A method of concentrating particles in a liquid-particledispersion by adsorptive bubble separation, the method comprising:introducing the liquid-particle dispersion into a vessel at a pointbelow a surface of a liquid contained therein; introducing a gas intothe vessel at a point below the introduction point of theliquid-particle dispersion; forming bubbles comprising a gas-particleagglomerate; directing the bubbles to the surface of the liquid with oneor more baffles so as to form a floating froth, wherein the froth isenriched in particles and wherein the one or more baffles direct therising bubbles and falling dislodged particles to the perimeter of thevessel; directing the froth towards a froth collection launder in aregion of lower surface area than the region where it formed; andcollecting the froth in the froth collection launder.
 18. The methodaccording to claim 17, wherein the froth collection launder is locatedsubstantially in the center of the vessel. 19-20. (canceled)
 21. A frothflotation device of the type wherein froth floating in a vessel flowsinto a collection launder for collection, the improvement comprising:incorporating one or more baffles in the vessel to direct rising bubblesand falling dislodged particles to the perimeter of the vessel; andincorporating into the froth flotation device a vacuum generating deviceto pull and/or collapse froth through the collection launder.
 22. Thefroth flotation device of claim 21, wherein vacuum is generated by apump, compressor, aspirator, venturi, eductor, and/or blower used toprovide gas introduced into the vessel for preparation of bubbles forfroth.
 23. A froth flotation device of the type wherein froth floatingin a vessel flows into a collection launder for collection, theimprovement comprising: one or more baffles in the vessel for directingrising bubbles and falling dislodged particles to the perimeter of thevessel; and a collection launder centrally located within the vessel.